1. Field of the Invention
This invention relates to data telemetry and particularly as applied to seismic exploration.
2. Discussion of the Prior Art
The term "light" as used herein is intended to encompass visible light as well as radiation in other parts of the spectrum that may be propagated by optical waveguides.
In seismic exploration, acoustic signals are injected into the earth from a location at or near the surface. The injected acoustic signals propagate downward and are reflected from structures in the subsurface. The reflected acoustic signals return to the earth's surface where they are detected by a plurality of seismic sensors or sensor groups. The sensors are normally deployed along a surveyed line and are spaced tens or hundreds of feet apart along a length of cable. As many as a thousand or more sensors may be distributed over such a cable which may be three miles or more long.
The acoustic signals received by the sensors are transformed into electrical or optical signals and transmitted to a central signal-recording station. In accordance with conventional seismic surveying practice, each sensor is located along the survey line at increasingly greater distances from the recording station.
Technologically-old seismic data-transmission systems include a plurality of sensors disposed along a cable at predetermined spacings. Each sensor transmits data to a remote recording device through a physically separate transmission line. Because of increasingly longer cables having many more sensors, separate transmission-line cables became physically cumbersome to handle.
Time-division multiplexed (TDM) systems were developed to reduce the large number of transmission lines to a mere handful and yet maintain sensor distribution density. TDM sensors are spaced at predetermined distances along each transmission line such that the data transmitted by each sensor to the recording device will not overlap data from adjacent sensors on the same line. The TDM data propagating on each line are received at the recording device and demultiplexed in channel-sequential or sample-sequential order well known in the art.
In electrical telemetric systems, the recording station polls each sensor in sequence and identifies each individual sensor by an address to that sensor. Alternatively, various clocking schemes have been developed whereby each sensor has a response to one or more clocking signals emitted by the recording station.
In optical telemetric systems, the recording station launches a light pulse of predetermined width into a trunk transmission line. As the light pulse propagates down the trunk line, a portion of the light pulse energy enters each sensor where it is modulated and passed back into the trunk line but in the opposite direction. The time delay due to the propagating distance between sensors provides a time window during which each sensor can transmit data, thus providing a time-division multiplexed optical signal. The modulated light pulses from each sensor are sequentially received and demultiplexed at the recording station. Individual sensors are recognized by the arrival sequence of their signal.
The above telemetric systems have several disadvantages. Electrical telemetric systems require complicated circuits so that each sensor or sensor group is able to recognize its particular address or clock pulse. Optical systems do not require addressed sensors. However, signal attenuation is a problem resulting from the large number of optical couplers connecting the sensors to the trunk line. Signal loss is compounded when the system is comprised of several sections.
It is an object of this invention to provide a new multiplexing method and system for reducing signal loss in optical telemetric systems.
It is another object of this invention to reduce the number of sensors per telemetry channel while maintaining sensor density along the line of survey.
It is yet another object of this invention to provide a two-stage, multiplexed telemetric system wherein each channel has a time-division multiplexed signal propagating therethrough and wherein the output of the sensors on each channel are staggered with respect to each other.